The invention relates to an FM-receiver comprising a frequency-locked loop which includes, in succession, a voltage-controlled oscillator, a mixer circuit connected to an aerial input, an IF-portion comprising an IF-filter, an FM-detector, a loop filter and a loop amplifier for adjusting the transfer characteristic of the frequency-locked loop. The loop amplifier is connected to a control input of the voltage-controlled oscillator for feedback of the modulation signal of the received FM-signal, the filter comprising a first low-pass filter.
Such an FM-receiver is known from the Netherlands Patent Application No. 7906602 which has been laid open to public inspection.
In the known FM-receiver, when being correctly tuned to a wanted transmitter signal, the FM-aerial signal having an average frequency RF is amplified and mixed down in the mixer circuit to a low, average intermediate frequency IF by means of the oscillator signal which has an average frequency OF. Simultaneously, by the feedback of the modulation in the frequency-locked loop, the frequency deviation of the received FM-aerial signal is compressed, for example by a factor of 5 from 75 KHz to 15 KHz. This reduces considerably the foldover distortion of the FM-signal in the IF-portion, which is the result of the comparatively low average intermediate frequency IF.
The low intermediate frequency IF in combination with the compression of the frequency deviation makes it possible to realize the known FM-receiver in integrated circuit form. However, for an adequate signal processing a number of requirements must be satisfied. For a selectivity which is effective in the most critical circumstances, and an adequately wide FM-IF-passband the IF-filter must be of a higher order, for example of the 4.sup.th order. In addition, in practice the stability of the frequency-locked loop is guaranteed only when, within the passband of the closed loop--that is to say the frequency range in which the loop gain is equal to one--the phase shift of the modulation signal in the loop is less than 180.degree.. This phase shift is predominantly effected in the IF-filter, the FM-detector and the first low-pass filter and limits significantly the number of choices as regards the order, the bandwidth and the class of the filters. A further restraint is the requirement that for an effective compression of the frequency deviation the bandwidth of the open loop, that is to say the bandwidth of the first low-pass filter must include at least a considerable portion of the modulation signal.
In practice these requirements can only be satisfied by a certain choice of filter parameters, when the bandwidth of the modulation signal is of the order of magnitude of that of an FM-mono signal.
With modulation signals having a bandwidth of the order of the stereo multiplex signal (53 KHz) the requirements as regards compression of the frequency swing deviation, selectivity and stability result in conflicting filter parameters. Thus, for an effective compression of the frequency deviation of FM-stereo signals, the bandwidth of the first low-pass filter shoulld amount to 40 to 45 KHz and the open loop gain within this bandwidth must be approximately 12 dB. This results in a passband of the closed loop of 160 to 180 KHz when the first low-pass filter has a first order roll-off of 6 dB/octave. Owing to the first low-pass filter the phase shift within the pass-band of the closed loop is at its maximum at the 160 to 180 KHz limit frequency of this passband. With the given order and loop gain this maximum phase shift amounts to approximately 90.degree.. In order to satisfy the stability requirement the phase shift produced by the IF-filter and the FM-detector within this 160 to 180 KHz passband must be less than approximately 90.degree.. For a 4.sup.th or higher order IF-filter this is only realizable at very large bandwidths. Such very large bandwidths are impermissible for an effective IF-selectivity. Consequently, the prior art FM-receiver is not suitable for receiving and processing FM-stereo signals.